The invention relates to a vaporizing crucible for vacuum vapor coating systems, especially for electron beam heating, consisting of a plurality of segments which are assembled and held together in contact with one another by pressure plates.
A vaporizing crucible of the kind initially described, for heating by the passage of electrical current, is known through German Pat. No. 1,276,421. In this crucible the object was to join together a relatively large number of segments to produce a crucible of great length to make possible the vapor coating of the entire width of, for example, moving bands of material. In the operation of vaporizing crucibles of this kind, however, it has been found that unacceptably high electrical contact resistances develop at the joints, resulting in a local overheating of the crucible material in those areas. This results in a premature end of the useful life of the crucible. Moreover, it has been observed that unequal temperature distribution over the entire length of the crucible produces relative displacements of the segments in relation to one another. The reason for this is probably temperature differences which occur during the heating operation, and differences in thermal balance over the full length of the crucible, which in turn are caused by uneven heating over the length of the crucible and varying heat losses due to radiation at different points on the crucible, which are difficult to control. As a result, a constant friction occurs at the confronting faces of the crucible segments, producing an attrition of the crucible material with a simultaneous relaxation of the contact pressure between them. The problem of contact resistance at the joints between the segments occurs, of course, only when the segments consist of a material which by nature is electrically conductive, or of a refractory material which has been made electrically conductive by means of special additives. In this case the train of segments is clamped between electrical contacts and raised to the required vaporization temperature by the direct passage of electrical current through it.
In the case of vaporization crucibles heated by electron beams the contact resistance between the segments is of no importance as far as heating is concerned. Vaporizing crucibles for electron beam heating have hitherto been made always in one piece, it being necessary, when crucible material is used which has a poor electrical conductivity in the cold state, to provide for the removal of the charged particles so as to enable the vaporization to be performed continuously. If the crucible material is insufficiently conductive, the build-up of an electrostatic charge on the content of the crucible would reflect the electrons, so that, under certain circumstances, controlled heating becomes impossible. An attempt has been made to solve this problem by drilling holes in the floor of the crucible and providing a grounding wire threaded meander-wise through the holes for the purpose of grounding the contents of the crucible. This method, however, resulted either in the melting of the wire and the interruption of the circuit, or in a leakage of the vaporized material through the holes, and in an undesirable chilling of the crucible content by the contact thus produced between it and the necessary water-cooled support on which it is mounted. Furthermore, the material being vaporized is converted by the metal of the grounding wire to an alloy which may have undesirable characteristics.
In addition to the electrical problems, however, the negative effects of differences in the thermal expansion of the individual segments still persist, especially in the case of electron beam heating, inasmuch as in this case much greater temperature gradients occur than in resistance-heated crucibles. That is, the power is applied at the focal point, so that temperatures are produced locally which are 2 to 3 times as high as the temperatures on the outside surface of the crucible. Furthermore, in the case of the electron beam heating of vaporizing crucibles it is difficult to prevent stray electrons or reflected electrons from impinging upon the crucible material, producing a very intense additional heating of the crucible material in the area of such impingement. For the reasons given above, the previously known segmented crucible is unreliable both when nonconductive materials are used and when conductive materials are used. Furthermore, due to the heat capacity and the poor thermal conductivity of the ceramic material, when the apparatus is opened to the outside atmosphere for charging. a thermal shock is produced which results in severe tensions in the material and hence in cracking.